Flat panel displays are widely used in a variety of applications, including computer displays. One type of device well suited for such applications is the field emission display. Field emission displays typically are matrix addressable displays that include a generally planar substrate having an array of projecting emitters. In many cases, the emitters are conical projections integral to the substrate. Typically, the emitters are grouped into emitter sets where the bases of the emitters are commonly connected.
A conductive extraction grid is positioned above the emitters and driven with a voltage of about 30-120 V. The emitters are then selectively activated by providing a current path from the bases to ground. Providing a current path to ground allows electrons to be drawn from the emitters by the extraction grid voltage. If the voltage differential between the emitters and the extraction grid is sufficiently high, the resulting electric field extracts electrons from the emitters.
The field emission display also includes a display screen mounted facing the substrate. The display screen is formed from a glass plate coated with a transparent conductive material to form an anode biased to about 1-2 kV. A cathodoluminescent layer covers the exposed surface of the anode. The emitted electrons are attracted by the anode and strike the cathodoluminescent layer, causing the cathodoluminescent layer to emit light at the impact site. The emitted light then passes through the anode and the glass plate where it is visible to a viewer.
The brightness of the light produced in response to the emitted electrons depends, in part, upon the rate at which electrons strike the cathodoluminescent layer, which in turn depends upon the magnitude of current flow to the emitters. The brightness of each area can thus be controlled by controlling the current flow to the respective emitters. By selectively controlling current flow to each of the emitters, the light from each area of the display can be controlled and an image can be produced. The light emitted from each of the areas thus becomes all or part of a picture element or "pixel."
Typically, current flow to the emitters is controlled by controlling the voltage applied to the bases of the emitters to produce a selected voltage differential between the emitters and the extraction grid. The electric field intensity between the emitters and the extraction grid is then the voltage differential divided by the distance between the emitters and the extraction grid. The magnitude of the current to the emitters then corresponds to the intensity of the electric field.
One problem with the above-described approach is that the response of the emitters to applied grid and emitter voltages may be non-uniform. Typically, this is caused by variations in the separation between the emitters and the extraction grid across the array, which causes differences in the electric field intensity for a given voltage difference. Consequently, the rate of electron emission, and thus the light intensity, may vary across the display.
Additionally, the intensity of light emitted can vary in response to the activation-to-emission response of the cathodoluminescent layer. As used herein, activation-to-emission response refers to the amount of light energy emitted by the catiodoluminescent layer or a region of the cathodoluminescent layer for a given level of electron flow. A high activation-to-emission response then corresponds to emission of a relatively large amount of light energy for a given emitter current.
Variations in the cathodoluminescent layer thickness, or in the chemical makeup of the cathodoluminescent layer can cause variations in the activation-to-emission response of the cathodoluminescent layer, such that the intensity of emitted light can vary across the array even with a uniform distribution of electron flow. This is particularly common in color displays, in which the cathodoluminescent layer includes different phosphor formulations that produce light at red, green, or blue wavelengths. Each of the phosphor formulations can have a different activation-to-emission response. For example, red phosphor formulations typically have a higher activation-to-emission response than green phosphor formulations. The light intensity for a given current level will therefore be higher for red pixels than green pixels in the absence of corrective measures.